1. Field of the Invention
This invention relates to a rotation control device suited to be used in video tape recorders or the like.
2. Description of the Related Art
In recent years, the development of imaging instruments, such as video tape recorders (hereinafter abbreviated to VTR), is remarkable. Taking for example the VTR, besides the normal recording and reproduction modes, it is possible to choose a high speed search mode, still reproduction mode, slow reproduction mode, etc., where the tape runs with selection of a plurality of different speeds from the normal one for reproduction.
In the high speed search mode, for example, because the tape speed is far faster than in the normal reproduction, if the speed of rotation of the rotary head drum remains the same as in the normal reproduction, a discrepancy arises between the speeds of the head and the record track on the tape. And, this discrepancy causes shift of the frequency of the reproduced signal with the result of a chromatic deviation, and shift of the horizontal synchronizing frequency being reproduced, which tends to have the horizontal synchronism in the television receiver, or monitor display put out of order.
To solve these problems, measure is taken to correct the rotational frequency of the rotary head drum so that the speed of the head on the rotating drum and the record track on the running tape coincide with each other.
FIG. 1 is a block diagram of a drive control circuit for the rotary head drum and the capstan in, for example, the 8 mm VTR.
In the figure, a system control circuit 1 for controlling all the coordinations of the system is constructed with a CPU, a ROM having control programs stored therein, a RAM for memorizing every item of control information, etc. An electric motor 2 rotates a rotary head drum. From the motor 2 for rotating the drum, or from the rotary head drum, a PG pulse generator 3 detects its phase of rotation. An FG pulse generator 4 produces a pulse signal representing the speed of rotation of the motor 2 for rotating the drum. The PG pulse produced by the PG pulse generator 3 is supplied to a phase comparator circuit 6 where it is compared in phase with a reference signal (the vertical synchronizing signal Vsync produced by a synchronous separation circuit 12 to be described later, or the output ref.sub.1 of a frequency divider 13) to produce a phase error signal. The FG pulse produced by the FG pulse generator 4 is supplied through an amplifier 7 to a speed comparator circuit 8 where it is subjected to frequency-to-voltage conversion to produce a speed error signal. The phase error signal and speed error signal are added by an adder 9 and then fed back through an amplifier 10 to a drive circuit 11 for driving the motor 2 for rotating the drum. Thus, a speed servo and a phase servo are operated so as to always maintain constant the speed and phase of rotation of the motor 2 for rotating the drum.
The synchronous separation circuit 12 separates the vertical synchronizing signal Vsync from the video signal. The frequency divider 13 divides the frequency of a standard signal from a crystal oscillator (not shown) or the like into a predetermined value (for example, the vertical synchronizing frequency) to produce a reference signal ref.sub.1 which is supplied to the phase comparator circuit 6. A switch S.sub.1 moves in response to selective setting of the recording and reproduction modes. Switching is operating in such a manner that during recording, the vertical synchronizing signal of the inputted video signal is supplied to the phase comparator circuit 6, while during reproducing, the reference signal output from the frequency divider 13 is supplied to the phase comparator circuit 6.
When the speed of transport of the tape is altered by the system control circuit 1 (for example, increased N times,) a phase correction circuit 14 and a speed correction circuit 15 cause, respectively, the phase comparator circuit 6 and the speed comparator circuit 8 to have respective such new target values that the phase and speed of the rotary head drum match the new speed of transport of the tape.
Meanwhile, in a system for controlling the driving of a capstan for moving the tape, there are an electric motor 16 for driving the capstan, and an FG pulse generator 17 responsive to rotation of the motor 16 for driving the capstan for producing a pulse signal. This FG pulse produced by the FG pulse generator 17 is supplied through an amplifier 18 to a phase comparator circuit 19 where it is compared with a reference signal ref.sub.2 (the output of a frequency divider 20) to produce a phase error signal. The frequency divider 20 divides the frequency of a standard signal output from an oscillator (not shown) to produce the reference signal ref.sub.2 which is supplied to the phase comparator circuit 19. A frequency divider 21 capable of varying the frequency division ratio divides the frequency of the FG pulses output from the amplifier 18 by "N" based on the command from the system control circuit 1. The frequency-divided FG pulses output from the frequency divider 21 is subjected, for example, to F-V conversion by a speed comparator circuit 22 to produce a speed error signal. A detector circuit 23 detects, for example, the 4-frequency pilot signal for ATF (for Auto Tracking Finding) (or the control signal CTL) out of the reproduced signal and puts it to balanced modulation with a reference frequency to produce predetermined two beat signals. An ATF control circuit 24 performs a computation of the output signal of the detector circuit 23 to detect a tracking error of the head relative to the record track and produces an output representing an amount of correction. A switch S.sub.2 interlocks with the switch S.sub.1 to select the output of the phase comparator circuit 19 or the output of ATF control circuit 24 depending on the selected one of the recording and reproducing modes. An adder 25 adds the speed error signal output from the speed comparator circuit 22 and the selected one by the switch S.sub.2 of the phase error signal output from the phase comparator circuit 19 and the tracking error signal output from the ATF control circuit 24. The composite error signal produced by the adder 25 is fed back through an amplifier 26 to a drive circuit 27 for the motor for driving the capstan. Thus, the speed and phase at which the capstan is driven is controlled. That is, during recording, the phase comparator circuit 19 is selected to connect with the adder 25 by the switch S.sub.2. Hence, the motor 16 for driving the capstan is controlled by comparing its phase with the phase of the reference signal. During reproducing, the ATF control circuit 24 is selected to connect with the adder 25 by the switch S.sub.2. Hence, the motor 16 for driving the capstan is controlled in accordance with the tracking error signal.
Now suppose such a VTR is set in normal reproducing mode, then the switches S.sub.1 and S.sub.2 take their PB positions. In the rotary head drum control system, therefore, the composite error signal produced from outputs of the phase comparator circuit 6 and the speed comparator circuit 8 is used to control the rotation of the motor 2 for driving the rotary head drum. For this purpose, the phase comparator circuit 6 is supplied with the reference phase from the frequency divider 13. Also, the phase correction circuit 14 and the speed correction circuit 15 are given a control command from the system control circuit 1 so that the reference signal corresponding to the normal reproduction speed is supplied to the phase comparator circuit 6 and the speed comparator circuit 8.
In the capstan control system, the composite signal produced from the speed error signal output from the speed comparator circuit 22 and the tracking error signal output from the ATF control circuit 24 are used to control the rotation of the motor 16 for driving the capstan.
With this condition, when the reproduction mode is set to, for example, high speed or N times faster search, the system control circuit 1 supplies the frequency divider 21 with a command of changing its ratio of division of the frequency to 1/N. Thereupon, the speed comparator circuit 22 controls the speed at which the capstan is to rotate in feedback manner so as to cancel that change. Eventually, the speed of rotation of the capstan becomes N times as fast. Thus, the N times faster mode starts to operate.
Meanwhile, even the rotary head drum control system is supplied from the system control circuit 1 with a command for causing the phase correction circuit 14 and the speed correction circuit 15 to supply a reference signal representing the amount of correction of the phase and the amount of correction of the speed in correspondence to the N times faster rotation to the phase comparator circuit 6 and the speed comparator circuit 8. In such a way, the speed of rotation of the rotary head drum is N times increased.
Also, the outputs of the phase correction circuit 14 and the speed correction circuit 15 representing the amount of correction of the phase and the amount of correction of the speed respectively, if it is an analog servo circuit, takes the form of an increase or decrease of the constant voltage or current, or if it is a digital servo circuit, the form of a shifted frequency of the speed and phase reference signal by changing the ratio of division of the frequency in the reference oscillator.
However, according to the method described above, if the number of times the speed in the high speed search mode is faster than the normal recording or reproduction mode is as small as a few times, the time from its command until the tape transport speed reaches the value the command instructs and becomes stable, and the time until the speed of rotation of the rotary head drum locks in at the value the command instructs are both relatively short. Since the discrepancy between the speeds of the tape and the head is small, it does not cause large influence on the disorder of the horizontal synchronism of the television set or the monitor display.
However, to realize a high speed search of higher times, the ramp time from the moment at which the "N times search" command has been given to the moment at which the motor for driving the capstan and the motor for driving the drum actually lock in can no longer be ignored. Hence, the use of the above-described control method leads to a possibility of occurrence of disorder of the horizontal synchronizing, color disappearance, and a color deviation.
Also, even in the stationary state of the high speed search, it is necessary that the motor for driving the capstan is locked in always just at N times. Hence the servo system is required to have an extremely high precision accuracy of high speed control. This causes the control algorithm to change to a more complicated form and its cost to become rapidly higher.
It is to be noted that as the related art on the speed control of the rotary head drum or the like to be used in the present invention, mention may be made of those disclosed in U.S. Pat. No. 4,517,501 concerning the speed, phase servo of the rotary head drum and in U.S. patent application Ser. No. 369,011 filed on Jun. 15, 1989 where upon detection of when the rotary head drum has locked in phase, the reproduced synchronizing signal from the recording medium is used to establish synchronism.